This invention relates to quenching of molten slag discharged from a furnace, and particularly to the molten residue discharged from a solid waste disposal furnace having a non-submerged taphole of the type disclosed in U.S. Pat. No. 3,729,298.
Over the past decade, much emphasis has been placed on developing processes for the efficient disposition of solid waste to obviate the problems and difficulties of conventional waste disposal methods. The two most widely employed waste disposal methods have been landfilling and incineration. Landfilling operations have been found to be an ephemeral solution to the problem, since not only are available landfill areas becoming rapidly depleted, but the use of waste as landfill has contributed to the contamination of underground water creating possible health hazards. The alternative waste disposal technique, incineration, normally results in objectional air pollution. Moreover, since large gas volumes, stemming from the use of air as the combustion supporting gas would need to be treated, removal of pollutants is unduly expensive.
One process developed to circumvent the aforementioned problems of these conventional disposal methods is described in U.S. Pat. No. 3,729,298 -- Anderson, "Solid Refuse Disposal Process and Apparatus." This patent discloses a process referred to herein as the Anderson process for disposing of refuse while simultaneously producing a useful gaseous product and an inert solid residue, comprising the steps of (a) feeding refuse into the top portion of a vertical shaft furnace, (b) feeding an oxygen containing gas into the base of said furnace, (c) pyrolyzing the organic portion of the refuse, (d) fluidizing the inorganic portion of the refuse, (e) discharging the gaseous products from the top of said furnace, and (f) tapping the molten residue, i.e. the fluidized inorganic material from the base of said furnace.
In U.S. Pat. No. 3,806,335 -- entitled, "Process for Preventing Solidification in Refuse Converter Taphole," Anderson described a process for continuously withdrawing the molten residue, comprising slag and molten metal formed in the hearth of a refuse converter. This disclosure discloses a method for ensuring that the molten residue formed by the waste disposal process is maintained in a molten condition in the hearth, and additionally that the taphole is kept sufficiently hot to permit the molten slag to form from the hearth into a collection vessel without solidifying.
The refuse disposal process of U.S. Pat. No. 3,729,298 operates preferably at super-atmospheric pressure in the shaft furnace. U.S. Pat. No. 3,806,335 alludes to the problems related to maintaining the taphole open in furnaces operating under such a positive pressure condition. Whereas, this difficulty may be mitigated by operating the taphole in a submerged condition, U.S. Pat. No. 3,806,335 discusses why such a mode of operation is impractical. More precisely, because the refuse composition and, accordingly, the amount of molten residue produced fluctuates, it is difficult as well as impractical to ensure that the molten material always covers the taphole opening. Therefore, operation with a submerged taphole cannot be depended upon to successfully maintain the hearth sealed from the atmosphere.
Because of the evident problems in operating a taphole in a submerged condition, it has been suggested by Anderson in U.S. Pat. No. 3,720,298 as well as in U.S. Pat. No. 3,741,136 to Stookey, that the taphole be operated in a non-submerged condition, and that it be suitably enclosed to prevent the hearth from communicating directly with the surrounding atmosphere. This was done by linking the taphole to the water quench bath by a slag chute or duct. By suspending the slag duct into the water bath, a liquid seal is formed, preventing the hot gases within the hearth from flowing to the surrounding atmosphere.
The water bath also provides a suitable means for quenching the hot molten residue flowing from the hearth. Quenching the slag causes it to break into small granules, resulting from the rapid cooling of the molten slag from 2500.degree. F. to a solid residue at about 200.degree. F. When the slag is granulated in this manner, the particles are typically small and brittle varying between 1/16 and 1/4 of an inch in size. This granulated slag is generally transported from the quench basin by a conveyor for subsequent use, for example, as a landfill.
In the above-described process employing a water bath for quenching of the molten residue, it is essential to quench in hot water in order to prevent slag explosions. Unfortunately, however, while the use of an enclosed slag duct and a hot water quench bath solves one problem it creates another. It has been found that if used in a solid waste disposal process such as the earlier mentioned Anderson process, cooling of the hot molten residue in the water bath generates a considerable amount of steam which exerts a substantial cooling effect on the tap and hearth area. This cooling effect raises the viscosity of the slag and may cause plugging of the taphole. The seriousness of the tap and hearth cooling problem can be appreciated better when one considers that one pound of slag when quenched in water at 212.degree. F. will vaporize about one pound of steam. If this steam is allowed to enter the slag tap, which is at a temperature of about 3000.degree. F., it will require about 1,600 BTU to heat the pound of steam from 212.degree. F. to 3000.degree. F. Failure to add this extra heat to the hearth will cause it to cool rapidly, particularly in the slag tap causing the slag to freeze in the tap.